The present invention relates to a composite sheet and sealed container excellent in preserving ability and resistance to deterioration of contents contained therein.
Hitherto, as preserving containers made of plastics, there have been used those which comprise a gas barrier layer comprising an ethylene-vinyl alcohol copolymer or a vinylidene chloride copolymer, both sides of which are laminated with polyolefins such as polypropylene and polyethylene. These containers have been widely used as having both the gas barrier properties and sealing properties with a lid. However, with the progress of investigation on deterioration of contents contained in the containers, it has been found that in the case of containers having an innermost layer comprising a polyolefin, the innermost layer adsorbs flavor of the contents or an odor component of the innermost resin layer soaks into the contents. Thus, an innermost layer formed of other than polyolefins has been demanded.
As containers having these flavor characteristics, those which have an inner layer comprising polyethylene terephthalate, ethylene-vinyl alcohol copolymer and polycarbonate have been proposed (JP-A-3(1991)-244 and JP-B-55(1980)-13907). Among them, the former can be used for hot-filling of contents or low-temperature sterilization (lower than 100.degree. C.) of contents, but cannot be used for high-temperature sterilization (retorted foods) because the innermost layer is softened with heat. On the other hand, as resins covering these defects, use of the latter polycarbonate has been proposed (JU-B-5(1993)-8127, JU-A-61(1986)-115626, and JP-A-60(1985)-68938). However, since polycarbonate is a heat resistant resin, it is poor in heat sealability, and if usual heat sealing is to be carried out, 230.degree. C. or higher of sealing platen temperature is needed and in this case, the lid material sometimes melts and deteriorates and thus a sufficient sealing cannot be obtained at temperatures at which the lid material is not deteriorated. Furthermore, in combination with other resins, when a multi-layer sheet is subjected to vacuum forming, the forming cannot be satisfactorily performed because the polycarbonate is a heat resistant resin.